Sideprojects are usually about learning new things + following your passion.
I absolutely LOVE music. Its like air to me. I dont go more than 10 minutse without a song on my stereo or headphones. I also absolutely love to code, and constantly want to find new ways to get excited about learning how to code.
I think a passion for music correlates w/ the coder mentality pretty frequently. Thats why you see so many efforts aligned with that trend if you ask me.